It is often advantageous in electronic circuits to amplify a complex signal having multiple sinusoidal components (e.g., a multi-tone signal) without introducing distortion products into the amplified signal. The physical characteristics of contemporary amplifiers are such, however, that amplifiers do, in fact, inject distortion into the signal, especially when high power amplification is performed. In addition, linear power amplification is being required by today's complex communications systems for both multi-tone types of amplifiers and digital modulation schemes such as CDMA. Linearity is usually achieved at the expense of efficiency, especially at high power levels where it is a significant impact to system design and cost. The use of feed forward technology in linear power amplifiers is a way of achieving good linearity with improved efficiency, but adds to the cost and complexity of typical amplifiers. In addition to linearity requirements, many amplifier systems such as paralleled linear power amplifiers or other paralleled amplifier matrix systems require amplifier modules that have constant gain and phase for power combining efficiency.
In most amplifier designs, the majority of the distortion products are generated by the high power output or driver transistors. These stages cost the most money and dissipate the majority of the power of the amplifier and are consequently sized and biased to achieve the best efficiency possible. Many high power transistors operating with 8 dB or more backoff, which is required by multi-carrier and digital modulations, have IM performance that can vary with bias. The bias point for best IM performance will change with power, frequency, or temperature, and may vary from transistor to transistor. LDMOS type of transistors in particular have very sensitive IM versus bias curves, as discussed in Microwaves & Rf, March 1998, Vol. 37, No. 3, pages 69-80. Thus, a need exists for an apparatus and method for linear power amplification wherein the bias of the amplifier is dynamically adjustable.